My assumption is that the "ideal" situation is steady shot fires .5 sec before auto shot is ready to fire. Also, Steady shot is fired as soon as each GCD is up. This would mean steady had a 1 sec cast and auto had a 1.5 sec cool down. This is neglecting player error and latency problems.
What I really want to know is what does and does not effect the cast time of steady shot. Does serpents swiftness? IAofT? Dragonspine trophy? Abbacus of Violent odds? Bloodlust? Passive item haste?
How do you know if it will effect steady shot or not?
Last patch they made haste effects work with channeled spells, i.e. Mind Flay and Drain Life. So it should work fine with volley.
From patch notes:
Spell Haste: The amount of spell haste rating needed per percentage of haste has been increased substantially. However, spell haste will now affect channeled spells, increasing the rate of their effects and decreasing their total duration. The same amount of damage/healing per casting will occur, but will take less time.
Spell haste now affects channeled spells, it says nothing about physical haste. Every other reply on this thread so far is correct.